DAVID LUKEHURST

ASTRONOMICAL TELESCOPE MAKER

Mirror Care

From the moment a newly aluminised mirror is unwrapped, it will inevitably start to accumulate a fine layer of dust. Although this dust detracts from the pristine appearance of the mirror it will have little effect on performance unless the mirror becomes very dirty.

The main effect of the dust is to cause a slight loss of image contrast due to the light scattering effect of any dust or grime. It will also slightly reduce the reflectivity of the surface.

It is best to take every reasonable effort to stop the mirror from getting dirty in the first place by keeping it properly covered when not in use and storing the telescope in a clean environment. A clean Chamois leather can be used to cover the main mirror of a Newtonian. It should be kept warm whilst the telescope is in use and then replaced over the mirror when it is put away. Based on experience, this will protect the mirror and will not harm the aluminium coating even if it is directly in contact with it, so long as it is dry.

It is also advisable to avoid taking a cold telescope into a warm room as this is likely to result in condensation forming on the mirrors. If the telescope can be stored somewhere that is at outside temperature, it will also mean that when the telescope is taken outside to be used, it does not have to cool down before it will give steady images.

Dust can be carefully flicked off with a clean lens brush or blown off with a clean air duster. Care should be exercised when using aerosol duster as they can blow out liquid propellant which will mark the surface (although this will not cause any permanent damage).

Aluminised mirrors should never be rubbed with anything in an effort to clean off grime. This will inevitably scratch the coating. This sort of scratching, consisting of many fine parallel scratches, acts like a diffraction grating and will give rise to clearly perceptible bands of light radiating from stellar images.

If the mirror has become very dirty, a careful washing may be desirable. However, a new aluminium coating remains quite soft for some months and any cleaning should be avoided at this time. Over time, a very thin, hard, transparent layer of aluminium oxide forms and this protects the underlying metallic aluminium.

The mirror can be carefully immersed in lukewarm slightly soapy water and allowed to soak for a few minutes. It can then be wiped under the surface of the water very carefully with cotton wool. It should then be carefully rinsed with lukewarm water followed by a final rinse with lukewarm distilled water. It should not be rinsed with cold water as, in an extreme case, it could crack due to thermal shock. It should then be stood on edge and any drops of water remaining on the aluminised surface should be picked off with the corner of a piece of tissue. If done carefully, this cleaning process will clean the mirror very effectively and without harming the coating.